1. Field of the Invention
The present invention relates generally to a small-size two cycle combustion engine of a kind mainly used as a drive source for a work machine or equipment such as, for example, a brush cutter.
2. Description of the Prior Art
Two cycle combustion engines of the kind referred to above conventionally have scavenging passages for supplying an air/fuel mixture into the engine combustion chamber. In those two cycle combustion engines, the scavenging passages are formed in an engine cylinder block simultaneously with casting of the engine cylinder block. Formation of respective scavenging ports at downstream ends of the scavenging passages has hitherto required the use of a slide die slidable in a direction radially of the cylinder block, thus rendering the die assembly to be complicated and costly.
In order to alleviate the foregoing problems, Japanese Examined Utility Model Publication No. 58-43616, for example, discloses the use of scavenging-passage-defining wall members separate from the engine cylinder block, but incorporated in the cylinder block to define the scavenging passages. Specifically, the scavenging-passage-defining wall members have their opposite side edges inserted from below into corresponding engagement grooves, which are defined in the cylinder block so as to extend in a direction parallel to the cylinder axis, i.e., the longitudinal axis of the cylinder bore in the cylinder block and are then fixed in position to define the respective scavenging passages partitioned from the cylinder bore.
However, considering that in the two cycle combustion engine of the conventional type referred to above, an air/fuel mixture within the crankcase, which contains air and fuel mixed still insufficiently, is introduced into the scavenging passages during the scavenging stroke, fuel contained in the air/fuel mixture is apt to deposit on inner surfaces of the scavenging passages during the flow of the air/fuel mixture through the scavenging passages to thereby form layers of fuel pitch. Once the layers of fuel pitch so deposited come to have a substantial thickness, they may depart from the inner surfaces of the scavenging passages to form fuel droplets, which are subsequently introduced into the combustion chamber. Accordingly, the fuel mixed insufficiently with the air, which is so introduced into the combustion chamber, will be eventually burned and this leads to reduction in combustion efficiency. Also, it may occur that since the air/fuel mixture introduced into the combustion chamber through the scavenging ports is not sufficiently and satisfactorily controlled in direction of flow towards the combustion chamber, the blow-off of the air/fuel mixture cannot be suppressed effectively.
On the other hand, the two cycle combustion engine disclosed in the above mentioned prior art document is found to be advantageous in that the cost of manufacture thereof can be reduced, but has a problem associated with the use of the scavenging-passage-defining wall members that are made of the same material as that of the cylinder block, for example, an aluminum alloy or spring steel, and separate from the cylinder block. Specifically, since those scavenging-passage-defining wall members are fixed in position solely by means including the engagement grooves defined in the cylinder block for receiving therein the opposite side edges of the scavenging-passage-defining wall members, one or some of the scavenging-passage-defining wall members may accidentally fall or depart downwards from the associated engagement grooves at the time the cylinder block is connected with the crankcase, resulting in a possible reduction of the assemblability of the combustion engine.